I would say it's an obvious ADVANTAGE of open source software/firmware: if the original creator stops working on it, you can always (in theory: can you afford it?) hire someone to do the work for you, or band together with other users to do so: if a normal company went under (as we've seen happen to traditional steno companies?), then you'd be up the creek without a paddle.
And at any rate, it's a non-issue: the firmware is extremely simple and is using technology that has been supported for decades. We're using steno protocols from the traditional steno machines of the 90s, and the USB serial support in operating systems such as Windows is (I believe) there primarily to support data-gathering equipment also from the 90s that's still in use in research labs and on factory floors all over the world. None of this is going to change or go away any time soon. And it's simple enough that a skilled programmer could implement it in a day or two from scratch if need be: I did that for fun at one point.
Also, all of the firmware is open source and most of it is QMK based. So it's the same code across all the hobbyist steno machines. And QMK is the largest open source firmware that's used on most ordinary mechanical keyboards as well, not just steno.
Bottom line, this isn't the kind of situation where you will ever need to change or upgrade the firmware. And if by some corporate insanity operating systems do drop support for this broad class of devices 10 years down the road, the entire open steno community will be in the same boat and somebody will fix it.
I don't know who brought up that concern to you but (I'm sorry) they clearly have NO idea what they're talking about.
(I do want to point out that if you're planning on going into this as a career, you almost certainly will want to buy a professional steno machine eventually; but at that point it'll be a business expense and you'll have specific reasons for wanting one, and a hobbyist steno board will get you through the learning process more cheaply)
--Josh